Revolutionary increases in speed and reliability of microprocessors has been successfully achieved in the past 60 years. The faster and higher performance of microprocessors is based on increased transistor density. As originally proposed, Moore's law stated that the number of transistors in semiconductor devices or integrated circuits (ICs) would double approximately every two years. This prediction has been realized, largely due to device scaling, characteristic of fine pitch interconnects. Copper interconnects are now routinely used with the minimum feature size down to 65 nm; 45-nm node can be found in some commercial devices. However, the electrical resistivity of copper interconnects increases with a decrease in dimensions due to grain-boundary and electron surface scattering. As current density further increases, the electromigration issue for metal interconnects becomes more severe.
In view of these problems, carbon nanotubes (CNTs) have been proposed as a future interconnecting material due to their ultra-high current carrying capacity (109 A/cm2), thermal stability and high resistance to electromigration. Today, the main challenges of CNT interconnects (circuits) are: 1) purification of metallic or semiconducting CNTs; 2) selective positioning of CNTs; and 3) effective and reliable contacts at CNT junctions. Although the purification issue has almost been addressed by recent development of various CNT separation methods, CNT positioning and contact reliability issues are still unaddressed.
In spite of electrical performances, increasing microprocessor performance is associated with an increased cooling demand; in other words, more efficient heat dissipation is required. It has been reported that a reduction in the device operation temperature corresponds to an exponential increase in reliability and life expectancy of a device. To control device temperature within operation limits is critical. Thermal Design Power (TDP, the maximum sustained power dissipated by the microprocessor) has in the past increased steadily with increasing microprocessor performance. Although multicore microprocessors should alleviate the growth in TDP with increased performance, thermal non-uniformity, usually referred to as the “hot spot” issue, where the local power density could be >300 W/cm2, must be paid more attention to in circuit design and operation. Hot spot issue makes the heat dissipation near the chip more difficult. Effective heat dissipation has become a key issue for further development of high performance semi-conductor devices. Development of novel thermal interface materials (TIMs) is crucial to meet thermal performance requirements for future generations of high-performance IC chips. For thermal management applications, the distinctive thermal properties of carbon nanotubes (CNTs) attract much attention and give rise to new opportunities in thermal management of microelectronic devices and packaging systems. However, CNT/substrate contact resistance at the interface is typically high.